(1) Field of the Invention
This invention relates to a sheath-core composite filament having highly electrically conductive properties, and to a process for the preparation thereof.
(2) Description of the Prior Art
Composite filaments are known comprising as one component an electrically conductive thermoplastic synthetic polymer including electrically conductive carbon black. These have been used widely as basic materials for imparting antistatic capability to fiber products such as carpets and garments.
Known typical composite forms are: the two-layer sheath-core type (U.S. Pat. No. 3,803,453) with an electrically conductive layer as the core and an electrically non-conductive layer as the sheath; the three-layer sheath-core type (U.S. Pat. No. 4,207,376) with an electrically conductive layer as the middle layer surrounded completely by an electrically non-conductive layer; the partially encapsulated side-by-side conjugate type (U.S. Pat. No. 3,969,559) in which an electrically non-conductive layer encapsulates partially an electrically conductive layer; the side-by-side simple conjugate type (U.S. Pat. No. 4,129,677) in which an electrically conductive layer extends along the periphery of the filament and the interface formed by the non-conductive layer and the conductive layer is convex/concave; and, the two-layer reverse sheath-core type (Japanese Patent Publication No. 82/25647) with an electrically conductive layer as the sheath and an electrically non-conductive layer as the core.
Composite filaments of the two-layer sheath-core type or the three-layer sheath-core type as above-mentioned are useful antistatic materials for use in fiber products, being widely used in antistatic carpets and antistatic garments, because they have good yarn forming properties in that the electrically conductive layer is not exposed on the filament surface and presents no problem of fibrillation due to composite interface peeling.
However, in a composite filament having an electrically conductive layer surrounded completely by an electrically non-conductive layer, such as a filament of the two-layer sheath-core type or the three-layer sheath-core type as above-mentioned, the level of surface electrical resistivity of which is about 10.sup.10 to 10.sup.11 .OMEGA., is unsuitable for use in static control and contamination control garments for clean rooms, which require a high surface electrical conductivity of about 10.sup.6 to 10.sup.7 .OMEGA.. Therefore, static control and contamination control garments employ a coating type electrically conductive filament in which the filament is coated with an electrically conductive resin including electrically conductive carbon black. However, such coating type electrically conductive filaments are disadvantageously uneven in thickness due to the coating, and are poor in weaving properties. Therefore, a need exists for a composite filament having a high electrical conductivity equivalent to that possessed by the coating type filament.
In the preparation of the foregoing partially encapsulated side-by-side conjugate type composite filament, which has an electrically conductive layer exposed on the filament surface and is manufactured by the side-by-side composite process, two components which are extremely different in melt viscosity must be conjugated side-by-side in order that the electrically conductive component of high viscosity is partially encapsulated by the electrically non-conductive component of low viscosity. Therefore, in order to minimize the scatter of composite forms and keep the proportion of exposure of the electrically conductive layer constant, it is necessary to control strictly the melt viscosities of both components. As a result, the filament is disadvantageously difficult in industrial production.
The foregoing side-by-side simple conjugate type composite filament has also further disadvantages in that the interface formed by the non-conductive layer and the conductive layer is extremely easy to peel off, and the filament is readily fibrillated during use. Therefore, the filament is unsuitable for use in static garments for clean rooms.
In addition, the foregoing two-layer reverse sheath-core type composite filament involves several problems. Since carbon black of high concentration is present all over in the filament surface layer, carbon black readily falls out during the production processes, resulting in extensively stained production apparatus, and the filament per se is black in color reflecting no light, whereby it cannot be seen with the naked eye. Therefore, such filament is very difficult to produce industrially; in fact, no filament of this type is manufactured in the industry.
Any of the stated conventional type composite filaments in which an electrically conductive layer is exposed on the filament surface, involves problems in yarn formation or yarn quality and is unsatisfactory for industrial production.